1. Field of the Invention
The present invention relates to a projection type video display in which an image writing to a read light valve that generates projection image light is performed by using light.
2. Description of the Related Art
A projection type video display in which an image writing to a read light valve which generates projection image light is performed by using light is known (refer to a pamphlet of International Laid-Open WO 2005/116719 and Japanese Unexamined Patent Application Publication No. 2004-94115, for example). As an example, a conventional projection type video display is shown in FIG. 1. White light emitted from a white light source 101 is converted to substantially parallel light by a parabolic reflector, and the parallel light is introduced into an integrator lens 102. The integrator lens 102 is constructed by a pair of fly's eye lenses 102a and 102b and each of pairs of lens parts leads light emitted from the white light source 101 to the whole surface of a read light valve described later. Light that has passed through the integrator lens 102 is introduced into a first dichroic mirror 105 after passing through a polarization converter 103 and a collective lens 104.
The polarization converter 103 is constructed by a polarization beam splitter array (hereinbelow, referred to as PBS array). Each of PBSs has polarization separation films and a retardation film (1/2λ plate). Each of the polarization separation films in the PBS transmits, for example, P-polarized light in light from the integrator lens 102 and changes the optical path of S-polarized light by 90°. The S-polarized light whose optical path is changed is reflected by the adjoining polarization film and goes out as it is. On the other hand, the P-polarized light which has passed through the polarization separation film provided on the front side (light-exit side) of the PBS is converted to S-polarized light by the retardation film and the S-polarized light goes out. That is, in this case, substantially all of light is converted to S-polarized light.
A first dichroic mirror 105 transmits first color light while reflecting second color light and third color light. The first color light which has passed through the first dichroic mirror 105 is reflected by a tilted reflecting mirror 106. The first color light reflected by the tilted reflecting mirror 106 is introduced into a transmission-type read light valve 131 for first color light via a lens 107. The first color light changes to first color image light as a result of passing through the read light valve 131. On the other hand, light reflected by the first dichroic mirror 105 is introduced into a second dichroic mirror 108.
The second dichroic mirror 108 transmits third color light while reflecting second color light. The second color light reflected by the second dichroic mirror 108 is introduced into a transmission-type read light valve 132 for second color light via a lens 109. The second color light changes to second color image light as a result of passing through the read light valve 132.
A first dichroic cube 112 is provided at a position where an optical path of the first color image light and an optical path of the second color image light cross each other. The first dichroic cube 112 transmits the first color light while reflecting the second color light. When the first color image light and the second color image light are incident on the first dichroic cube 112, they are directed to the same direction.
The third color light which has passed through the second dichroic mirror 108 is introduced into a transmission-type read light valve 133 for third color light via a lens 110. The third color light changes to third color image light as a result of passing through the read light valve 133. An optical path of the third color image light is changed by 90° by a reflecting prism 111.
A second dichroic cube 113 is provided at a position where the changed optical path of the third color image light and an optical path which is the optical path of the first color image light combined with the optical path of the second color image light cross each other. The second dichroic cube 113 transmits the first color light and the second color light while reflecting the third color light. When the first color image light, the second color image light and the third color image light are incident on the second dichroic cube 113, they are directed to the same direction. This causes full-color image light to be generated.
At the light exit side of the second dichroic cube 113 (beside a surface which emits the full-color image light), there is disposed a projection lens 114. The full-color image light emitted from the second dichroic cube 113 is projected onto a not-shown screen through the projection lens 114.
Next, an image writing optical system will be described hereinafter. The image writing optical system includes three UV (Ultraviolet)-LEDs (light-emitting diodes) 121A, 121B and 121C as image-writing light sources for respective colors. The peak wavelength of the UV-LEDs 121A, 121B and 121C are different from one another. UV-LEDs 121A, 121B and 121C are turned on in a time-sequential manner. UV lights emitted from UV-LEDs 121A, 121B and 121C are directed to the same direction by dichroic mirrors 120A and 120B. When the UV lights pass through a rod integrator 122, a surface illuminant with a uniform light intensity is formed on an exit surface of the rod integrator 122. The UV lights emitted from the exit surface are introduced into a polarization beam splitter 124 after passing through a relay lens group 123.
Particular polarized light (for example, P-polarized light) which has passed through the polarization beam splitter 124 is introduced into a write light valve (for example, LCOS (liquid-crystal-on-silicon) device) 125 which modulates write light (the UV light). The write light valve 125 generates images for respective colors in a time-sequential manner by a not-shown driver. That is, the driver causes the write light valve 125 to form a first image based on a first color video signal when the UV-LED 121A is turned on, next writes a second image into the write light valve 125 based on a second color video signal when the UV-LED 121B is turned on, and writes a third image into the write light valve 125 based on a third color video signal when the UV-LED 121C is turned on, for example.
The write light valve 125 generates image light by modulating the received particular polarized light. The image light is obtained as reflected light, and the reflected light is changed to the other particular polarized light (for example, S-polarized light). That is, when the particular polarized light is irradiated on the write light valve 125, image-writing light having the other particular polarized light (S-polarized light) is generated. The image-writing light emitted from the write light valve 125 is reflected on the polarization beam splitter 124. The image-writing light is introduced into the second dichroic cube 113 through an imaging lens group 126.
Each of the second dichroic cube 113 and the first dichroic cube 112 has wavelength selectivity for the UV lights from the UV-LEDs 121A, 121B and 121C. Three UV lights having different peak-wavelengths from are split, and the split lights (the aforementioned image-writing lights) are introduced into the respective read light valves 131, 132 and 133. Specifically, first image-writing light based on the first color video signal is irradiated on the read light valve 131, second image-writing light based on the second color video signal is irradiated on the read light valve 132, and third image-writing light based on the third color video signal is irradiated on the read light valve 133.
As disclosed in the pamphlet of International Laid-Open WO 2005/116719, each of the read light valves 131, 132 and 133 is composed of an OASLM (Optically Addressed Spatial Light Modulator) having a photoconductive effect. For example, with a configuration in which a liquid crystal layer is interposed between optically transparent electrode structures having a photoconductive effect, photoconductive effect changes in only its portion where light is irradiated, with the result that a state of application of a voltage to the liquid crystal changes in the portion where light is irradiated, changing a state of rotation of the liquid crystal.
The conventional projection type video display described above, particularly the projection type video display shown in FIG. 1, however, needs a large projection lens having a long back focal length. In a case where the projection lens is particularly designed in wide-angle lens specifications, a distortion aberration is likely to occur. Further, in a case where a chromatic aberration of magnification occurs in a projection optical system, a color blur may occur in a projected image on a screen.